exception - 为什么我的变量在 erlang try 中不安全？

Question

我是 Erlang 新手。我对下面的代码有问题:

try
    {ok, [Card]} = io:fread("Input card number: ","~d")
    catch
      error:K -> (Card=0);
      exit:K -> (Card=0);
      throw:K -> (Card=0)
    end,

我不知道为什么'变量'Card'在'try'中不安全。对此我能做什么？

Answer 1

像您这样的代码可以在 case 表达式中工作，但对于 try 来说，这是 Erlang 中变量绑定(bind)规则的异常(exception)。来自 the reference manual :

For the try expression introduced in Erlang 5.4/OTP R10B, variable scoping is limited so that variables bound in the expression are always 'unsafe' outside the expression. This is to be improved.

您可以这样编写代码:

Card =
  try
    {ok, [C]} = io:fread("Input card number: ","~d"),
    C
  catch
    error:K -> 0;
    exit:K -> 0;
    throw:K -> 0
  end,